lowers and uppers

Just purchased 3 stripped lowers- one rock river and two s&w. Do I have a ton of options when it comes to a build. Whats good for one good for the other type thing? It will be a while before I start any builds but I am exited. Trying to read thru everything on this and other sites alot of great info.

Also am trying to find some ar 10 lowers, having a little more trouble but am being patient. Waiting for some shows to come around. But I have been reading a little and was wondering if there is more of a problem finding uppers. Has anyone built an ar10 lately, if so where could I find.